Conventionally, an integral type converter and a successive comparison type converter have been known as an A/D converter.
An integral type converter counts a number of clock signals which have a predetermined period, and integrates the clock signal to generate a slope voltage so that the counting operation stops just when a slope voltage exceeds the input analog potential. The digital output is the content of the counter when the counting operation stops.
A successive comparison type converter counts a number of clock signals which have a predetermined period. The content of the counter is converted to analog form, which is compared with input signal. When the converted analog signal becomes equal to the input analog signal, the counter stops, and the output of the converted digital signal is the content of the counter just when the counter stops. This type of converter is used in an A/D converter included in a micro-computer chip.
The accuracy of an A/D converter depends upon a number of bit positions of a counter and a D/A converter. As for an A/D converter included in a 4-bits micro-computer chip, a counter and a D/A converter have 8 bit positions. The resolving power or the minimum spacing of measure in this case is 0.4% (=1/256). When temperature, and/or relative humidity is displayed in digital form with an accuracy of 0.1.degree. C. and/or 0.1%, an A/D converter must have the resolving power of at least 0.1% (=1/1000).
Therefore, a prior 8 bits A/D converter is not enough for displaying temperature and/or relative humidity with an accuracy of 0.1.degree. C. and/or 0.1%.
The increase of the bit of a counter and a D/A converter is not desirable for improving the resolving power, because it would increase the production cost of an apparatus.